The present invention relates to a semiconductor protection circuit for turning off a semiconductor switching element if overcurrent is carried to the semiconductor switching element when trouble such as short-circuit occurs to a power control circuit provided with the semiconductor switching element.
Generally, a power control circuit into which a semiconductor switching element, such as an IGBT, is incorporated is provided with a protection circuit for protecting the semiconductor switching element from overcurrent. This protection circuit detects a load current according to a change in emitter output for current detection or a change in the voltage drop of a current detection resistor in the semiconductor switching element, and shuts off a gate input voltage being applied to the semiconductor switching element if, for example, a high current (to be referred to as “short-circuit current” hereinafter) is carried to the semiconductor switching element following the occurrence of trouble such as short-circuit, thereby turning off the semiconductor switching element. Normally, response delay of about several microseconds exists between the occurrence of trouble such as load short-circuit and the operation of the protection circuit. For that reason, there is a possibility that the semiconductor switching element is broken before the gate current (or voltage) is shut off by the operation of the protection circuit. To deal with such a disadvantage, there is known hitherto use of an RTC (real-time control) circuit which responds faster than the protection circuit. The RTC circuit operates to suppress the output current of the semiconductor switching element to a certain level or less and prevents the semiconductor switching element from being broken before the protection circuit starts its operation.
FIG. 7 shows a part of the configuration of a power control circuit employing a semiconductor protection circuit provided with a conventional RTC circuit. In the power control circuit, a protection circuit 80 is connected to the gate of a semiconductor switching element 103 through an output stage 101 and a resistor 102 and an RTC circuit 90 is connected between the gate and the source (or between the base and the emitter) of the semiconductor switching element 103. The protection circuit 80 has an AND circuit 81, a flip-flop circuit 82, a comparator 83 and a reference voltage source 84. A voltage signal proportional to an output current from the semiconductor switching element 103 is inputted into this protection circuit 80. If an input voltage exceeds a voltage applied from the reference voltage source 84, the comparator 83 turns the signal inputted into the output stage 101 through the AND circuit 81 into an off state to thereby turn off the semiconductor switching element 103. In this case, however, an operating signal is transmitted by way of the comparator 83, the AND circuit 81 and the output stage 101 in this order. Due to this, large delay exists from the time the output current reaches a protection level until the semiconductor switching element 103 becomes inoperative.
If a motor is used and an inductance component becomes a heavy load, the output current of the semiconductor switching element 103 increases relatively mildly with a ratio of time×output voltage/load inductance. The delay of the protection circuit does not cause a problem. However, if a resistor or a capacity component is a main component such as load short-circuit, the output current of the semiconductor switching element 103 has a sudden increase. As a result, before the protection circuit 80 is actuated to shut off the current to the semiconductor switching element 103, the element 103 may possibly be broken.
The RTC circuit 90 normally has two resistor-potential dividing elements 91 and 92 dividing the voltage of a resistor 104 connected to the current detection output of the semiconductor switching element 103, an MOSFET 93 (or bipolar transistor), and a resistor 105 connected to the gate of the semiconductor switching element 103. As in the case of the protection circuit 80, a voltage signal proportional to the output current of the semiconductor switching element 103 is inputted into the RTC circuit 90. The voltage signal is inputted into the gate of the MOSFET 93 (or the base of the bipolar transistor) through the resistor-potential dividing element 91. If the semiconductor switching element 103 is normally turned on, a gate voltage sufficiently higher than the threshold voltage of the element 103 is applied to the element 103 up to saturation so as to sufficiently lower on-resistance. During a normal state in which the current carried to the semiconductor switching element 103 is equal to or lower than a rated value, the MOSFET 93 is turned off and the MOSFET 93 does not influence the operation of the semiconductor switching element 103. If the output current increases and the MOSFET 93 is turned on, then the gate voltage of the semiconductor switching element 103 decreases to thereby turn the semiconductor switching element 103 into an active operation state. Then, the on-resistance of the semiconductor switching element 103 increases and the output current decreases, accordingly. If the output current decreases, the input voltage of the RTC circuit 90 decreases, whereby the function of the RTC circuit 90 for decreasing the input voltage of the semiconductor switching element 103 deteriorates.
As can be seen, the RTC circuit 90 forms a kind of a negative feedback circuit and operates to suppress the output current of the semiconductor switching element 103 to be a certain value or less. Although the RTC circuit 90 only cannot turn off the semiconductor switching element 103, the circuit configuration of the RTC circuit 90 is relatively simple and operation delay with respect to the protection circuit 80 is small. If trouble such as load short-circuit occurs and the output current suddenly increases, the RTC circuit 90 operates prior to the protection circuit 80, suppresses the output current of the semiconductor switching element 103 and then the protection circuit 80 turns off the semiconductor switching element 103.
Meanwhile, if the RTC circuit 90 is used along with the protection circuit 80, the semiconductor switching element 103 may possibly be broken at the time trouble such as load short-circuit occurs. FIG. 8 is a graph showing a change in the collector current Ic of the semiconductor switching element 103 and a change in the current Isens of the current detection terminal of the semiconductor switching element 103 in accordance with the operation of the RTC circuit 90. As can be seen from FIG. 8, when the RTC circuit 90 operates, the output current of the semiconductor switching element 103 rapidly increases and decreases repeatedly to form an oscillation waveform. Due to this, the current distributions of the respective components of the semiconductor switching element 103 are not uniform, with the result that a phenomenon that a current is concentrated only in a part of the components of the semiconductor switching element 103 and the part of the components is broken.
Further, when the protection circuit 80 operates, the input current (or voltage) of the semiconductor switching element 103 has been decreased by the RTC circuit 90. Due to this, the semiconductor switching element 103 is turned off faster than usual and a change in output current per unit time (di/dt) becomes often larger. As a result, a surge voltage exceeding the withstand voltage of the semiconductor switching element 103 is generated by the electromotive force of wiring inductance and the semiconductor switching element 103 is broken.
To avoid the above-stated problems, it is necessary to optimize the operation timing of the protection circuit 80 and that of the RTC circuit 90. Nevertheless, since the optimum timing of the protection circuit 80 and that of the RTC circuit 90 vary according to the state of the load and that of the semiconductor switching element 103, it is difficult to prevent the semiconductor switching element 103 from being broken under all conditions.
The present invention has been made in consideration of the above technical problems and it is, therefore, an object of the present invention to provide a semiconductor protection circuit capable of swiftly shutting off a current carried to a semiconductor switching element and ensuring the prevention of the semiconductor switching element from being broken if trouble such as load short-circuit occurs.